It seems that pin1 (what I'll call the connection closest to the cord) is ground, and the tip (pin3 for posterity sake,) is about 8.6VDC when the headset is not in use. When it is in use, it drops to a steady 8.4x (<8.5) VDC. So, if I can rig up something to sense the drop in voltage to trigger a relay to turn on a light, I'll be golden... In fact, that's just what I'll do. I'll post my results, so we won't need the Plantronics equipment. I'll probably spend more money doing this, than just buying the darn thing, but oh well...

This is for the 3 pin 3.5mm standard audio cable plugged into the lifter or in my case the cable that goes from the headset to the phone.